1. Field of the Invention
This invention relates to a new alloy composition, and, more particularly, to a new alloy composition and a method of forming drive axle shafts having a minimum diameter of 1.70 inches and a minimum capacity of 30,000 pounds. 2. Description of the Prior Art
One of the most important considerations in selection or formulation of a carbon steel alloy for producing a high strength axle shaft is controlling the hardenability of the alloy. Proper hardenability in turn depends upon having an alloy with the proper carbon content, that is, a high enough carbon content to produce the minimum surface hardness measured on the Rockwell C Scale, R.sub.c, and a low enough carbon content to be able to control the hardening process without exceeding maximum desired surface hardness or penetration of hardness into the core of the axle shaft. Hardenability establishes the depth to which a given hardness penetrates, which can also be defined as the depth to which martensite will form under the quenching conditions imposed, that is, at a quenching rate equal to or greater than the critical cooling rate.
Modern day hardenability concepts had their origin around 1930 in the research laboratories of United States Steel Corporation. In 1938 the Jominy Test came into being in the laboratories of General Motors as a means of determining hardenability. The test consists of quenching the end of a one inch round bar and determining the hardness, R.sub.c, at 1/16" intervals along the bar starting at the quenched end. Grossmann at United States Steel pioneered the calculation of hardenability presenting it in a paper published in the Trans Am. Inst. Mining Met. Engrs., V. 150, 1942, pp. 227-259. Grossmann postulated that hardenability can be based on a bar of ideal diameter, DI, defined as a diameter in inches of a bar that shows no unhardened core in an ideal quenching condition, or further defining it to produce a 50% martensite structure at the center of the bar. The calculation of DI is presented in many metallurgical texts, for example, in "Modern Metallurgy for Engineers" by Frank T. Sisco, second edition, Pitman Publishing Company, New York, 1948 or in the text "The Hardenability of Steels--Concepts, Metallurgical Influences and Industrial Applications" by Clarence A. Siebert, Douglas V. Doane and Dale H. Breen published by the American Society of Metals, Metals Park, Ohio, 1977.
Basically, the critical diameter in inches, DI, is calculated by muliplying together the multiplying factor, MF, for all the elements found in a particular steel either as residuals or purposely added to the steel. For example, a SAE/AISI 1040 carbon steel, using the Grossmann data would have the following multiplying factors for a typical percentage as follows:
Carbon 0.39% MF, =0.23; manganese 0.68%, MF 3.27; silicon 0.11%, MF=1.08; nickel 0.12%, MF=1.05, chromium 0.04%, MF=1.09; molybdenum, 0.02%, MF=1.06. The ideal diameter is then calculated as DI=0.23.times.3.27.times.1.08.times.1.05.times.1.09.times.1.06 equals 0.98 inches. This would mean that an ideal diameter with a perfectly quenched steel would be 0.98 inches; thus, to insure proper hardenability, the maximum diameter of this shaft would be something less than 0.98 inches probably of the order of 3/4".
By utilizing the DI calculations, it can be determined what can be the maximum diameter of the shaft of a particular composition that will have a desirable hardenability profile with 50% Martensite at the center of the core.
It is well established that high manganese carbon steel compositions provide satisfactory hardenability because the manganese allows the carbon to penetrate into the core in solution with the iron to produce the desired martensite as quenched. A SAE/AISI 1541 medium carbon steel having 0.36-0.44% C and 1.35-1.65% Mn will have adequate hardenability for axle shafts with a maximum diameter of less than 1.7 inches to produce a load carrying capacity of less than 30,000 pounds. Axle shafts with a body diameter greater than 1.7 inches for axle load carrying capacities of 30,000, 34,000, 38,000 or 44,000 pounds, cannot be produced with a 1541 steel because the manganese cannot produce a desirable hardness profile into the core of the shaft resulting in at least 50% martensite at the center. A satisfactory solution to this problem is obtained by the use of trace percents of boron in the SAE 1541 steel denoting the steel as SAE 15B41. Such boron percentages, are typically in the range between 0.0005-0.003% boron.
With the use of boron in the steel to produce the proper hardenability profile, the risk of retaining residual stresses after forging the usual spline at one end and flange at the other end of the axle shaft is present. This can greatly reduce the fatigue life of the shaft, producing premature failure by stress cracking. This is true because the boron will precipitate out into the grain boundaries as boron nitride to product brittleness. To counteract this the boron nitride is driven out of the grain boundaries when the axle shafts are normalized by heating to above the transformation temperature and air cooling. This is a time consuming and very expensive process.